The present invention relates to a fuel cell system, and to a method of starting the fuel cell system.
A fuel cell is a power generation element for generating electric power by an electrochemical reaction between a fuel gas, such as hydrogen or methanol, and an oxidizer gas such as oxygen. The fuel cell, in which the reaction products generated at the time of power generation is water, has been attracting attention as a power generation element not causing environmental pollution, and the use of the fuel cell, for example, as a driving power source for driving an automobile has been tried.
Fuel cells are classified into various types by the kind of electrolyte or the like, and a representative type which has been known is a fuel cell using a solid polymer electrolyte as the electrolyte. The solid polymer electrolyte type fuel cell is promising as a driving power source for electronic apparatuses, since it can be reduced in cost, it can be easily reduced in size, thickness and weight, and, from the viewpoint of cell performance, it has a high output density. The solid polymer electrolyte type fuel cell usually uses hydrogen as the fuel, and, in addition, there have been developed those in which hydrogen to be used as the fuel is produced by modifying methanol or natural gas. In recent years, further, a direct methanol type fuel cell has been developed in which methanol is supplied directly to the fuel cell as a fuel, thereby generating electric power.
The direct methanol type fuel cell is operated at an appropriate operating temperature of 60 to 80° C. Therefore, it is necessary to raise the temperature of the fuel cell in starting the fuel cell from its stopped state, so that the fuel cell cannot be started instantaneously. For solving this problem, there is a method in which driving is conducted by use of an auxiliary battery such as a lithium ion battery until the starting of the fuel cell. In the case where the time until the starting is long, however, an auxiliary battery having an electric capacity suited to the long-time operation is needed. In addition, there is also a method in which heating by a heater or the like is used, but this method also requires electric power of an auxiliary battery.
Besides, there is a fuel cell system in which, at the time of starting, the concentration of an aqueous methanol solution as a fuel is raised, and the aqueous methanol solution permeated through an electrolyte film is brought directly into combustion with oxygen at a cathode (refer to, for example, Japanese Patent Laid-open No. 2003-520399). In addition, there is a method in which methanol (refer to, for example, Japanese Patent Laid-open No. Hei 5-307970) or an aqueous methanol solution (refer to, for example, Japanese Patent Laid-open No. 2002-075414) is supplied directly to a cathode, to be brought directly into combustion with oxygen.
However, in the fuel cell system described in Japanese Patent Laid-open No. 2003-520399, methanol fed from a single methanol tank provided in the system is sprayed in a circulating passage connected to an anode by use of a spray nozzle, and the spray amount is controlled according to the temperature of the power generation cell. Specifically, the concentration of an aqueous methanol solution supplied to the anode is controlled by the amount of methanol sprayed by the spray nozzle. In this system, however, the concentration of the aqueous methanol solution supplied to the anode cannot be lowered unless the methanol in the aqueous methanol solution is consumed in the power generation cell. Therefore, when the temperature of the power generation cell is raised, the switching from a high-concentration aqueous methanol solution to a low-concentration aqueous methanol solution cannot be instantaneously achieved even if the spraying of methanol from the spray nozzle is stopped. Furthermore, this system has the problem that an operation at an optimum concentration cannot always be realized. In the method described in Japanese Patent Laid-open No. Hei 5-307970, most of the reaction between methanol fed directly to the cathode and oxygen is concentrated in the vicinity of a methanol inlet port, and heat is locally generated at that portion, so that the temperature of the power generation cell as a whole becomes nonuniform. Therefore, even when the temperature of the power generation cell is raised above a predetermined temperature, an efficient reaction cannot be achieved because the temperature of the power generation cell as a whole is not uniform. Besides, the method described in Japanese Patent Laid-open No. 2002-075414 has the problem that water in the aqueous methanol solution fed to the cathode would adhere to the cathode. Since the power generation reaction takes place at the interface between electrode and electrolyte, the adhesion of water to the cathode reduces the amount of oxygen making contact with the cathode, thereby lowering the efficiency of the power generation reaction.